The invention relates to a flux which can be used for dry application, and to the use thereof as a brazing flux.
For many years, it has been known to braze together components of aluminium or aluminium alloys, in particular heat exchangers for the automobile industry, using fluxes based on alkali fluoroaluminate. In such a case, the flux is usually sprayed on to the heat exchangers as an aqueous suspension. In the presence of a brazing composition or of a braze-forming precursor such as silicon powder or potassium fluorosilicate, a stable, non-corrosive compound is formed upon heating the components to a temperature above the melting point of the flux. A process has indeed already been disclosed by published German patent application no. DE-OS 197 49 042 with which the waste water produced in this procedure can be recirculated. However, other process parameters are critical: the concentration of the flux slurry must be monitored, the heat exchangers must be dried before heating, the flux slurries, which are likewise recirculated, may pick up impurities. These disadvantages can be avoided if the flux is applied dry to the components to be joined. This is the case in the dry-flux process. Therein, the dry flux powder is applied to the components electrostatically. The advantage is that no slurries need to be produced, that the concentration of the slurry does not need to be monitored, that it is not necessary to provide a separate drying stage for the components, and that no waste water is produced.
It is an object of the present invention to provide an improved brazing flux based on alkali fluoroaluminate.
Another object of the invention is to provide a brazing flux which can be conveyed well pneumatically and can be sprayed well in the dry state.
A further object of the invention is to provide a brazing flux which adheres well to components on which it is sprayed and is therefore suitable for the method of dry application (dry fluxing).
These and other objects are achieved in accordance with the present invention by providing an alkali fluoroaluminate flux which can be used for dry application, said flux having a particle volume distribution lying within the area bounded by Curves 1 and 2 of FIG. 10.
In accordance with a further aspect of the invention, the objects are achieved by providing a process for joining components of aluminium or aluminium alloy, said process comprising applying a flux having a particle volume distribution lying within the area bounded by Curves 1 and 2 of FIG. 10 to the components to be joined, the flux being applied dry and electrostatically charged, and thereafter heating the components to braze them together.
The invention is based on the finding that the particle size or the grain-size distribution of the alkali fluoroaluminate fluxes has an influence on the pneumatic transport, the spraying ability and the adhesive power of the flux particles on the components. It was discovered that it is advantageous if smaller and larger particles are contained in the flux and the ratio thereof is subject to certain rules.
The flux composed of alkali fluoroaluminate according to the invention which can be used for dry application (dry fluxing) is characterised in that the volume distribution of the particles lies substantially within Curves 1 and 2 of FIG. 10. The particle-size distribution was determined by laser diffraction.